1. Field of the Invention
The present invention relates to load cells for measuring static and slowly fluctuating load. More particularly, the present invention relates to a variable reluctance load cell for measuring the load, or tension, on static devices in an environmentally hostile environment (e.g., determining mooring line tensions of offshore oil platforms).
2. Description of the Related Art
Offshore deepwater platforms utilize various mooring systems for stationkeeping. A number of offshore platforms e. g. spars, deep draft caissons, semisubmersibles and floating production, storage and offloading vessels (FPSOs) are equipped with a means to jack the mooring chain and maintain tension on the line, reducing the amount of slack. Passively moored platforms or vessels that are not required to move may not be equipped with a permanently mounted tensioning system. As the mooring lines experience fatigue and stretch, the platform can twist, leading to increased friction between the links, and accelerating fatigue and failure. The amount of tension on the mooring line determines the amount of slack, and consequently the amount of relative movement of the platform or vessel.
Fairleads are used to attach the mooring chain to the deck of the platform or vessel. In one configuration, a chain stopper latches the chain outboard of the fairlead and allows the stopper to rotate freely about two perpendicular axis. All motion change between the mooring line and the vessel occurs on proper bearing surfaces, and not between the fairlead and the chain.
Tension in the vertical chain leg between the fairlead and the deck level stopper, combined with the rotation of the fairlead caused by yawing of the platform or vessel, promote undesirable wear in the chain links.
Similarly, suspension bridges rely on large cables to maintain support for the bridge span. The amount of tension on the suspension cables is indicative of the stress placed on the cables, and the amount and rate of cable wear and or fatigue.
Various devices are available to measure the amount of tension, or applied tensile force, placed on fasteners and securing lines, including strain gauge bridges, differential transformers, capacitance sensors and variable reluctance load cells.
Mooring line tensions have been measured with instrumented chain links that employ strain gauged shear pins, strain gauges and strain gauge load cells. Generally, these devices have not been reliable for long term applications in hostile environments, for example, marine and aerospace environments. Strain gauges require adhesive attachment to the surface being measured. In non-controllable environments, strain gauges are subject to drift caused, for example, by adhesive breakdown, requiring recalibration. In environmentally hostile environments, the frequency of recalibration, repair or replacements becomes expensive, and may even be dangerous to perform.
Variable reluctance load cells use a variable reluctance transducer to measure force. For example, a core and winding can be used to sense changes in proximity to a cantilevered spring. Changes in inductance, caused by changes in the gap between the core and the spring, are reflected in the frequency of an oscillator circuit. In a previous load cell sensor utilizing a variable reluctance transducer, the load cell sensor responded primarily to forces along a pre-selected axis, while being relatively insensitive to both forces along axis transverse to the pre-selected axis, and to bending moments. Accordingly, the load cell sensors were mounted in the middle of the structure under observation to compensate for bending forces. Additionally, the load cell sensor was internal to a load-measuring unit, and contained all the components. Intrusion of contaminants into the sensor region could lead to premature aging of the components, including corrosion, making the readings unreliable.
Accordingly, there is a need for a load measurement device that is less prone to the various effects of exposure to hostile environments and can take into account effects of bending.